Ebola has now infected about 9,000 people and killed nearly 4,500, including 263 health care workers, the World Health Organization reported Wednesday. These numbers are probably an underestimate, WHO officials said. Disease transmission is still rampant in Liberia, Guinea and Sierra Leone, although in some parts of those countries the number of new cases is falling. Those improvements are welcome, but overall the numbers of cases and deaths continue to climb. Countries must work to stamp Ebola out entirely, said WHO assistant director-general Bruce Aylward in a call with reporters on Tuesday. “This is Ebola. It is a horrible, unforgiving disease. You’ve got to get down to zero” cases, he said. Senegal and Nigeria are poised to declare victory over the deadly virus. In August, a Guinean man got sick with the virus while staying with relatives in Senegal. No one else fell ill, and tests on September 5 showed the man is no longer infected. On Friday, Senegal will reach 42 days without a new case, a benchmark that WHO uses to determine whether a country is Ebola-free. Nigeria will reach that goal on Monday. Officials are tracking 891 contacts of Nigeria’s 20 Ebola patients. Those contacts have already remained disease-free well past the virus’s incubation period of 21 days.